python: add queue property to Jobspec

Problem: The jobspec system.queue attribute is as important as
duration, cwd, and other jobspec attributes, but the queue is not
currently treated as a special property of jobspec.

Add jobspec.queue setter and getter to match other standard Jobspec
class properties.

Author: grondo

src/bindings/python/flux/job/Jobspec.py

@@ -371,6 +371,25 @@ def duration(self, duration):
             raise ValueError("duration must be a normal, finite value")
         self.setattr("system.duration", time)
 
+    @property
+    def queue(self):
+        """
+        Target queue of job submission
+        """
+        try:
+            return self.jobspec["attributes"]["system"]["queue"]
+        except KeyError:
+            return None
+
+    @queue.setter
+    def queue(self, queue):
+        """
+        Set target submission queue
+        """
+        if not isinstance(queue, str):
+            raise TypeError("queue must be a string")
+        self.setattr("system.queue", queue)
+
     @property
     def cwd(self):
         """